Cyclic-adenosine monophosphate (cAMP) and cyclic-guanosine monophosphate (cGMP) function as intracellular second messengers regulating an array of processes in neurons. Intracellular cAMP and cGMP are generated by adenyl and guanyl cyclases, and are degraded by cyclic nucleotide phosphodiesterases (PDEs). Intracellular levels of cAMP and cGMP are controlled by intracellular signaling, and stimulation/repression of adenyl and guanyl cyclases in response to GPCR activation is a well characterized way of controlling cyclic nucleotide concentrations (Antoni, Front. Neuroendocrinol. 2000, 21, 103-132).
Phosphodiesterase 2A (PDE2A) is a dual substrate enzyme with higher affinity for cGMP although it may metabolize either cAMP or cGMP depending on the tissue. cAMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms, conveying the cAMP-dependent pathway. Although expressed in the periphery, the highest expression levels of PDE2A are in the brain. A recent immunohistochemical study demonstrated a consistent pattern of PDE2A expression in the brain across mammalian species including humans (Stephenson, et al. J. Histochem. Cytochem. 2009, 57, 933). The enzyme expression was shown to be prominent in regions associated with cognitive function and mood control, including the cortex, striatum, hippocampus, amygdala and the habenula.
The selective PDE2A inhibitor, Bay 60-7550, preferentially increases cGMP in primary neuronal cultures and hippocampal slices. Bay 60-7550 also increases long term potentiation (LTP) induction in rat hippocampal slices. Consistent with its biochemical and electrophysiological effects, Bay 60-7550 was found to be active in novel object and social recognition tasks (Boess, et al. Neuropharmacology 2004, 47, 1081). More recently, Bay 60-7550 was reported to reverse the deficit in object recognition produced by tryptophan depletion (van Donkelaar, et al. Eur. J. Pharmacol. 2008, 600, 98). These results are interesting in light of the PDE2 positive cells identified in the dorsal raphe, a region known to contain the cell bodies of the serotonergic neurons projecting to the forebrain (Stephenson, et al. J. Histochem. Cytochem. 2009, 57, 933). A similar study in aged rats demonstrated that the beneficial effect of Bay 60-7550 on object recognition could be reversed by a neuronal nitric oxide synthase (nNOS) inhibitor, suggesting that the effects of PDE2A inhibition in the central nervous system (CNS) are due to alterations in the levels of cGMP (Domek-Lopacinska and Strosznajder Brain Res. 2008, 1216, 68).
Recent studies indicate that PDE2A inhibition may also efficacy in the treatment of anxiety states (Masood, et al. J. Pharmacol. Exp. Ther. 2008, 326, 369; and Masood, et al. J. Pharmacol. Exp. Ther. 2009, 331, 699). Induction of oxidative stress in mice by depletion of central glutathione levels with buthionine sulfoximine (BSO) results in an increase in a number of anxiety-like behaviours assessed by open field time and the elevated plus maze assays. These effects were reversed by treatment with Bay 60-7550. Increased cGMP signaling, either by administration of the PDE2 inhibitors Bay 60-7550 or ND7001, or the NO donor detanonoate, antagonized the anxiogenic effects of restraint stress on behaviour in the elevated plus-maze, hole-board, and open-field tests, well established procedures for the evaluation of potential anxiolytics. These drugs also produced anxiolytic effects on behavior in non-stressed mice in the elevated plus-maze and hole-board tests. By contrast, administration of an NOS inhibitor, which reduces cGMP signaling, produced anxiogenic effects similar to restraint stress.
Phosphodiesterase 10A (PDE10A) is another dual-specificity enzyme that can convert both cAMP to AMP and cGMP to GMP (Soderling, et al. Proc. Natl. Acad. Sci. 1999, 96, 7071). PDE10A hydrolyses both cAMP and cGMP having a higher affinity for cAMP. PDE10A is expressed in the neurons in the striatum, n. accumbens and in the olfactory tubercle (Seeger, et al. Brain Research, 2003, 985, 113-126) and the thalamus, hippocampus, frontal cortex and olfactory tubercle (Menniti et al., William Harvey Research Conference, Porto, December, 2001). All these brain areas are described to participate in the pathomechanism of schizophrenia (Lapiz, et al. Neurosci Behav Physiol 2003, 33, 13) so that the location of the enzyme indicates a predominate role in the pathomechanism of psychosis. In the striatum, PDE10A is predominately found in the medium spiny neurons and they are primarily associated to the postsynaptic membranes of these neurons (Xie et al., Neuroscience 2006, 139, 597). In this location PDE10A may have an important influence on the signal cascade induced by dopaminergic and glutamatergic input on the medium spiny neurons two neurotransmitter systems playing a predominate role in the pathomechanism of psychosis.
Psychotic patients have been shown to have a dysfunction of cGMP and cAMP levels and their downstream substrates (Muly, Psychopharmacol Bull 2002, 36, 92). Additionally, haloperidol treatment has been associated with increased cAMP and cGMP levels in rats and patients, respectively (Leveque et al., J. Neurosci. 2000, 20, 4011). As PDE10A hydrolyses both cAMP and cGMP, an inhibition of PDE10A would also induce an increase of cAMP and cGMP and thereby have a similar effect on cyclic nucleotide levels as haloperidol. The antipsychotic potential of PDE 10A inhibitors is further supported by studies of Kostowski et al. (Pharmacol Biochem Behav 1976, 5, 15) who showed that papaverine, a moderately selective PDE10A inhibitor, reduces apomorphine-induced stereotypies in rats, an animal model of psychosis, and increases haloperidol-induced catalepsy in rats while concurrently reducing dopamine concentration in rat brain, activities that are also seen with classical antipsychotics. In addition to classical antipsychotics which mainly ameliorate the positive symptoms of psychosis, PDE10A also bears the potential to improve the negative and cognitive symptoms of psychosis.
Focusing on the dopaminergic input on the medium spiny neurons, PDE10A inhibitors by up-regulating cAMP and cGMP levels act as D1 agonists and D2 antagonists because the activation of Gs-protein coupled dopamine D1 receptor increases intracellular cAMP, whereas the activation of the Gi-protein coupled dopamine D2 receptor decreases intracellular cAMP levels through inhibition of adenylyl cyclase activity. Elevated intracellular cAMP levels mediated by D1 receptor signalling seems to modulate a series of neuronal processes responsible for working memory in the prefrontal cortex (Sawaguchi, Parkinsonism Relat. Disord. 2000, 7, 9), and it is reported that D1 receptor activation may improve working memory deficits in schizophrenic patients (Castner, et al., Science 2000, 287, 2020).
Further indication of an effect of PDE10A inhibition on negative symptoms of psychosis was given by Rodefer et al. (Eur. J Neurosci 2005, 21, 1070) who could show that papaverine reverses attentional set-shifting deficits induced by subchronic administration of phencyclidine, an NMDA antagonist, in rats. Attentional deficits including an impairment of shifting attention to novel stimuli belongs to the negative symptoms of schizophrenia. In the study the attentional deficits were induced by administering phencyclidine for 7 days followed by a washout period. The PDE10A inhibitor papaverine was able to reverse the enduring deficits induced by the subchronic treatment.
These convergent findings indicate that the inhibition of PDE2A and/or PDE10A may be therapeutic targets for the treatment of certain neurological and psychiatric disorders. Accordingly, the present invention relates to pyridine containing triazolopyrazines, to their preparation, to their medical use and to medicaments comprising them.